Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
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Procedure Declaration (AX51 & A251 only)
Ax51 provides procedures to implement the concept of subroutines. Procedures can be executed in-line (control “falls through” to them), jumped to, or invoked by a CALL. Calls are recommended as a better programming practice.
PROC / ENDP (AX51 & A251 only)
The PROC and ENDP directives are used to define a label for a sequence of machine instructions called a procedure. For the Philips 80C51MX and Intel/Atmel WM 251 architecture a procedure may have either the type NEAR or FAR. Depending on the type it is called with LCALL or ACALL (for NEAR) or ECALL (for FAR). Unlike C functions, assembler procedures do not provide local scopes for labels. Identifiers must be unique in A251 because the visibility is module wide. The format of the PROC/ENDP directives is:
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[ type ] |
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RET |
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name |
ENDP |
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where |
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name |
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is the name of the procedure. |
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type |
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specifies the type of the procedure, and must be one of the |
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following: |
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Type |
Description |
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none |
The type defaults to NEAR |
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NEAR |
Defines a near procedure; called with LCALL or ACALL. |
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FAR |
Defines a far procedure; called with ECALL. |
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Shaded directives and options are available only in AX51 and A251.
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Chapter 4. Assembler Directives |
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You should specify FAR if the procedure is called from a different 64KByte segment. A procedure normally ends with a RET instruction. The software instruction RET will automatically be converted to an appropriate machine return instruction. For example:
RET |
Return from a near procedure. |
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ERET |
Return from a far procedure. |
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Example |
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P100 |
PROC |
NEAR |
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RET |
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; near return |
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ENDP |
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P200 |
PROC |
FAR |
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RET |
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; far return (ERET) |
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ENDP |
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P300 |
PROC |
NEAR |
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CALL |
P100 |
; LCALL |
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CALL |
P200 |
; ECALL |
RET |
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; near return |
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ENDP
END
Shaded directives and options are available only in AX51 and A251.
Keil Software — A51/AX51/A251 Macro Assembler and Utilities |
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LABEL (AX51 and A251 only)
A label is a symbol name for an address location in a segment. The LABEL directive can be used to define a program label. The label name can be followed by a colon, but it is not required. The label inherits the attributes of the program or code segment currently active. The LABEL directive may therefore never be used outside the scope of a program segment. The syntax of that directive is:
name[:] LABEL [ type ]
where |
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name |
is the name of the label. |
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type |
specifies the type of the label, and must be one of the following: |
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Type |
Description |
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none |
The type defaults to NEAR |
4 |
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NEAR |
Defines a near label. |
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FAR |
Defines a far label; use ECALL or EJMP. |
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You should specify FAR if the label will be referenced from a different 64KByte segment. NEAR lets you refer to this label for the current 64KByte segment.
Example
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RSEG |
ECODE_SEG1 |
; activate an ECODE segment |
ENTRY: |
LABEL |
FAR |
; entry point |
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RSEG |
ECODE_SEG2 |
; activate another ECODE segment |
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EJMP |
ENTRTY |
; Jump across 64KB segment |
Shaded directives and options are available only in AX51 and A251.